1. Field of the Invention
The present invention is directed to a method of operating a ventilator system, of the type having a ventilator and a connection system for, when connected to a patient, conveying gas to and from the patient.
The present invention is also directed to a ventilator system of the type having an inspiration section for generating an inspiratory respiratory gas pattern, a connection system for, when connected to a patient, conveying gas to and from the patient, an expiration section for generating an expiratory respiratory gas pattern, a measurement system for measuring respiratory gas pattern parameters at different sites in the ventilator system and a control unit for controlling the operation of the ventilator system based on set operational parameters and measured parameters.
2. Description of the Prior Art
In the past 50 years, the development of ventilator systems (which in as used herein means all respirator/ventilator systems as well as anaesthetic systems) has made rapid progress. From initially using simple mechanical piston systems to impose breathing gas on the patient at every piston stroke, today's ventilator systems can be controlled to supply a breathing gas to a patient according to a number of different operating modes, a physician then being able to select the operating mode deemed most suitable for the patient.
A ventilator system can be described as a ventilator with a connection system for connecting the ventilator to the patient. One known ventilator is the Servo Ventilator 300, Siemens-Elema AB, Solna, Sweden. This ventilator is equipped with a very fast and accurate gas regulation system. In practice, this means that a gas flow can be generated with an optional respiratory gas pattern, as used herein, the term respiratory gas pattern refers to pressure and flow characteristics over time. Pressure and flow in any given respiratory gas pattern can exhibit predefined variations over time.
Even if the regulatory system is capable of generating a gas flow which corresponds almost exactly to the target respiratory gas pattern, it is not certain that the gas flow received by the patient has the target respiratory gas pattern. This because of the interposed connection system, which influences the respiratory gas pattern.
The connection system can i.a. include tubes, humidifiers, dehumidifiers and bacterial filters. Flow resistance in gas lines and other components in the connection system influences the respiratory gas pattern in one way. The total volume taken up by the connection system influences the respiratory gas pattern in another way. This because gases are highly compressible. The influence to which the respiratory gas pattern is subjected by the connection system changes the pattern of gas flow with respect to delay and morphology (morphology here referring to variations in pressure and flow over time).
Attempts have previously been made to compensate, at least to some extent, for the influence of the connection system. For example, the operational manual for the aforementioned Servo Ventilator 300, AG 0593 3.5, Siemens-Elema AB, 1993, pp. 94-98, describes compensation for the connection system's compressible volume. Compensation in this case means that the physician must set a larger minute volume for the breathing gas to be supplied to the patient in order to ensure the delivery to the patient of the target minute volume. Here, the physician is forced to make the calculations required to achieve the necessary compensation. The calculation example on page 98 in the operational manual, meant for an adult patient, shows that minute volume has to be increased by 2.5 1/m when the target minute volume was 7.5 1/m. Compensation naturally varies from case to case. The need for compensation depends in particular on the configuration of the connection system. The calculation example, however, does provide an indication of the compensation needed for minute volume.
There are also other known ventilator systems offering compensation, either by a physician or by a programmed automatic function. The compensation mainly entails a determination of the connection system's compressible volume, for instance the Puritan Bennet, 7200 Series Microprocessor ventilator, option 30/40, part number 20522A, March 1986.
Determination of compressible volume, however, does not really indicate how a respiratory gas pattern is actually influenced and altered by the connection system. As noted above, respiratory gas pattern refers to pressure and flow variations over time. If the flowing gas is viewed as a gas column passing through the connection system, it will be realized that even simple compression of the gas column changes slopes of the non-constant parts of the column and, in particular, the pressure and flow variations over time. Thus, compressible volume does not indicate anything about, for instance, the way in which a target pressure increase the gas column is influenced on its way to the patient's respiratory system. Determining compensation for the connection system's compressible volume therefore does not supply sufficient information for the use in calculating compensation of the respiratory gas pattern. As already noted, the flow of gas is also delayed in the connection system. It may even be, that different parts of the connection system imposes different delays on the respiratory gas pattern.
In conjunction with both the diagnoses and treatment of disorders in the respiratory system (primarily the lungs) of a patient, determination of the lung's various mechanical parameters is desirable. Determination of resistance and compliance is especially important. Roughly speaking, compliance can be determined in ways similar to determinations of compressible volume in the connection system of the known ventilator systems. A particular problem, however, is that the connection system's influence on gas flow is not fully known in conventional ventilator systems, so determination of the lungs mechanical parameters is even more uncertain. Moreover, the properties of these mechanical parameters can also influence the respiratory gas pattern.
In conjunction with the development of increasingly more accurate and exact gas generating systems in the ventilator field, the ability to determine and take into account factors for which compensation previously could not be made is also more desirable.
By using complex mathematical methods, models representing the influence of each component in the connection system can be assumed and a model for the entire connection system can be calculated mathematically and used as an overall compensating model for the connection system. Such models, however, require that the transfer function for each component be verified not only for the component itself but also when interacting with other components in the connection system. Since components are manufactured by several manufacturers, each component from each manufacturer must be tested and verified as to its mathematical model. In practice, a physician must then program the ventilator system for each new configuration of the connection system.
Another possibility is to regard the entire connection system as an unknown transfer system, having an unknown transfer function. In an earlier Swedish patent application SE 9500275-4, corresponding to co-pending U.S. application Ser. No. 08/588,684 filed Jan. 16, 1996 ("METHOD AND APPARATUS FOR MAINTAINING A DEFINED RESPIRATORY GAS FLOW PATTERN TO A SUBJECT BY IDENTIFYING A TRANSFER FUNCTION OF THE CONNECTION SYSTEM," Castor et al.), assigned to the same assignee as the present application, such a system is described. A test gas pulse is generated by the ventilator and the resulting gas pulse responses are measured at different sites in the ventilator system. By using the resulting gas pulse response and the test gas pulse in mathematical methods of calculation, for instance Box Jenkins model structures or similar, a transfer function can be calculated for the entire connection system. This requires, however, the presence of fairly strong computer calculation capabilities.